Where to put `implicit none` in Fortran

Question

Do I need to put implicit none inside every function and subroutine?

Or is it enough to put it at the beginning of the module containing these functions and subroutines?

Or is it enough to put it at the beginning of the program that is using these modules?

From observation of other's working code, implicit none is included in all these places. I am not sure if this is done redundantly because removing implicit none from subroutines still compiled and produced the same output.

By the way, I am using gfortran fortran 90.

Answer 1

The implicit statement (including implicit none) applies to a scoping unit. Such a thing is defined as

BLOCK construct, derived-type definition, interface body, program unit, or subprogram, excluding all nested scoping units in it

This "excluding all nested scoping units in it" suggests that it may be necessary/desirable to have implicit none in each function and subroutine (collectively, procedures) defined in a module. However, inside procedures contained within a module there is a default mapping based on the host scoping unit. So, with implicit none in the module it isn't necessary to have that in contained procedures.

This host scoping unit rule applies equally to internal programs. This means that implicit none in the main program covers all procedures contained in it; and that the same applies for internal programs of module procedures. Block constructs see this also, and the implicit statement isn't even allowed within one of these.

However, external functions/subroutines will not inherit implicit behaviour from a program or module, and modules don't inherit it from programs/other modules which use them. This clearly makes sense as the implicit typing must be known at compile time and be well defined regardless of their ultimate use.

Further, one cannot do

implicit none
use somemodule

end program

An implicit statement must follow all use statements.

This host scoping unit rule notably doesn't apply to interface bodies. IanH's answer motivates that exception, but it's an important enough thing to re-stress. It has caused much confusion.

module mod
 implicit none

  interface
    subroutine external_sub()
      ! The default implicit typing rules apply here unless there is an implicit
      ! statement, such as implicit none.  Those from the module aren't in force here.
    end subroutine
  end interface

end module

Regarding the test of removing implicit none from a subroutine: if the code is valid with implicit none then it must be valid and identical without that statement. All entities must be explicitly declared in the former case, so no implicit rules would be applied in the latter.